1. Field of the Invention
This invention relates to a clamp-on voltage probe which is adapted to measure the amplitude of voltage pulses within the ignition wire of an automobile. As a result of the novel features of the clamp-on voltage probe, the accuracy of the voltage measurement is improved by 300 percent or more over prior art voltage probes. The clamp-on voltage probe is quickly attached over and detached from the insulated ignition wire of an automobile without disturbing the wire and also without disturbing the electrical characteristics of the ignition system.
2. Description of the Prior Art
Prior art voltage probes which are used to measure the ignition pulses in automotive systems suffer from many defects. Some probes are adapted to be connected in line with the ignition wire and require the ignition wire to be removed from the ignition system in order to connect the probe. These probes, although satisfactory for some applications, alter the electrical characteristics of the system and tend to change some fault symptoms. For example, an ignition wire termination which is faulty could be damaged when inserting the in-line probe so as to require repair of the connection and removal of the fault before diagnosis. Also, an undamaged ignition wire could be damaged when inserting the in-line probe.
Prior art clamp-on probes suffer from defects such as minimum dielectric thickness which result in probe dielectric breakdown and probe inaccuracy. Also the clamp-on probes which are commercially available do not accurately position the ignition wire with respect to the probe pickoff plates, resulting in inaccuracies in the voltage measurement.
In order to construct an improved voltage probe which will accurately measure the voltage in an insulated wire without disconnecting the wire, it is necessary to overcome two variables which result in inaccuracies, namely, the dielectric change in the insulation of the wire from wire to wire, and the changes in diameter and shape which commonly occur along the length of the wire. The change in wire dielectric may be overcome by providing a low pickoff plate capacitance which is much less than the wire capacitance and which will effectively reduce or cancel out variations in the dielectric capacitance of the ignition wire. The changes in wire diameter can be controlled by making the radius of the voltage probe much greater than the radius of the wire. An additional error in the voltage measurement may be induced by electrical fields created by the wire itself or by other wires in the locality which produce their own electrical fields thus charging up the capacitance plate by other means than the coupling between the wire whose voltage is being measured and the capacitive plate. This latter problem may be overcome by shielding of the capacitive plate.
The present invention overcomes deficiencies in the prior art voltage probes and provides improved accuracy by means of its unique mechanical and electrical design. In one embodiment, the probe is specifically adapted for use with automobile ignition wires, that is, 7 millimeter wire, while in a second embodiment the probe is adapted for use with wires of varying diameter. The voltage probe produces its improved accuracy by using three concentric cylindrical pickoff plates to which are connected a twisted shielded pair transmission wire. In both embodiments the probe is designed to maintain the ignition wire securely at the center of the probe and concentric with the pickoff plates.